Recently, with the active development of electric vehicles, storage batteries, robots, satellites, and the like, along with the dramatically increasing demand for portable electronic products such as laptop computers, video cameras, mobile phones, and the like, research and development for high-performance secondary batteries capable of repeatedly charging and discharging has been actively made.
Currently, nickel-cadmium batteries, nickel-metal hydride batteries, nickel-zinc batteries, lithium secondary batteries, and the like are used as commercial secondary batteries. Among them, lithium secondary batteries have little to no memory effect in comparison with nickel-based secondary batteries, and thus lithium secondary batteries are gaining a lot of attention for their advantages of free charging or discharging, low self-discharging, and high energy density.
A lithium secondary battery generally uses lithium oxide and carbonaceous material as a positive electrode active material and negative electrode active material, respectively. The lithium secondary battery includes an electrode assembly in which a positive electrode plate and an negative electrode plate respectively coated with the positive electrode active material and the negative electrode active material are disposed with a separator being interposed between them, and an exterior, namely a battery case, which seals and accommodates the electrode assembly together with an electrolyte.
Generally, a lithium secondary battery may be classified into a can type secondary battery where the electrode assembly is included in a metal can and a pouch type battery where the electrode assembly is included in a pouch of an aluminum laminate sheet, depending on the shape of the exterior.
In these days, secondary batteries are widely used not only for small-sized devices such as portable electronic devices but also for middle-sized or large-sized devices such as vehicles and power storages. In particular, along with exhaustion of carbon energy and increased interest on environments, the public attention is focused on hybrid vehicles and electric vehicles over the world, including USA, Europe, Japan and Korea. In such hybrid vehicles and electric vehicles, one of the most important parts is a battery pack which gives a driving force to a vehicle motor. A hybrid vehicle or an electric vehicle ensures better fuel efficiency and exhausts no or less pollutant substances since a driving force for a vehicle is obtained by charging/discharging a battery pack. Due to various advantages such as above, more and more vehicle users select such hybrid vehicles and electric vehicles.
A battery pack of the hybrid-electric vehicle or the electric vehicle generally includes a plurality of secondary batteries, and the plurality of secondary batteries are connected to each other in series or in parallel to enhance capacity and output. A general battery pack, for example a battery pack for a vehicle, includes a cell assembly in which a plurality of secondary batteries are stacked, and a pack housing for accommodating the cell assembly in an inner space thereof. In addition, in order to use the battery pack, the battery pack should be electrically connected to an external device by means of a connection member such as a connection wire, and for this connection, a connection terminal may be provided to the battery pack.
The connection terminal may be formed with various shapes. As a representative shape of the connection terminal, a thread may be formed at an outer side of the terminal so that a connection member is coupled to an outer side of the connection terminal, and a coupling member such as a nut is coupled thereon so that the coupled state between the connection terminal and the connection member may be fixed. At this time, since a thread is formed on the outer side of the connection terminal to have a bolt shape, the connection terminal is also called a terminal bolt.
However, a torque may be applied to the terminal bolt while a coupling member such as a nut is being coupled, and the terminal bolt should not be easily damaged or deformed due to the torque. In particular, since a hybrid-electric vehicle or an electric vehicle may be frequently exposed to strong vibrations in use, the coupling member such as a nut should be strongly coupled to the terminal bolt, and thus a stronger torque may be applied to a terminal bolt of a battery pack for a vehicle. Therefore, there is needed a battery pack which is not damaged or deformed by a strong torque. Further, a battery pack used for a vehicle is a middle-sized or large-sized battery pack with high voltage and high capacity. Thus, damage or deformation of the terminal bolt or peripheral parts may lead to more serious damages such as fire, explosion or electric short. Therefore, a battery pack for a vehicle may be regarded as having more strict requirements on torque-related performance for a terminal bolt or the like.